Peel, pit, and cube avocados. Puree in blender or food processor. Add stock, buttermilk, lemon juice, peel, and garlic and process until smooth. Salt and pepper to taste. Cover and refrigerate until well chilled. Rinse and drain shrimp. Ladle soup into chilled bowls and garnish with shrimp and parsley. Serve with a wedge of lemon.